In a classical computer we can read out the state of all the bits in the computer at any time. The following 3 bits are in a definite state of 101, and can be read one-zero-one in a classical computer. Due to the property of superposition of a qubit in a quantum computer; it is impossible to determine the exact state of the computer. This means, we cannot determine exactly which superposition state a qubit is in because of their ability to switch from one state to another. Rather, we can only obtain partial information about the state of the computer. This makes it extremely difficult to build a quantum computer because we cannot determine the exact state at any time. Therefore control measures to control the state of qubits must be created, which are called control devices.
Interesting fact: The state of a qubit can be represented by a wavefunction.
How Quantum Computer Work - http://computer.howstuffworks.com/quantum-computer.htm
Quantum Computer - http://www.sciencedaily.com/news/computers_math/quantum_computers/